Method for manufacturing surface acoustic wave device and surface acoustic wave device

ABSTRACT

A method for manufacturing a surface acoustic wave filter device includes a step of forming grooves in one principal surface of a piezoelectric substrate, a step of embedding a metallic film in the grooves to form IDT electrodes, a step of performing a process of removing a portion of the piezoelectric substrate from the one principal surface of the piezoelectric substrate, thereby forming a recessed portion including the bottom surface in which the IDT electrodes are embedded, and a step of bonding a cover member to the piezoelectric substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a surfaceacoustic wave device that is preferably used as a surface acoustic waveresonator or a surface acoustic wave filter which utilizes a surfaceacoustic wave, such as Rayleigh wave, Love wave, or Leaky wave, and asurface acoustic wave device. More particularly, the present inventionrelates to a method, in which a step of forming an air gap in a portionin which a surface acoustic wave is excited is improved, a method formanufacturing a surface acoustic wave device, and a surface acousticwave device.

2. Description of the Related Art

In the related art, the miniaturization of mobile communicationequipment, such as mobile phones, has been in high demand. For thisreason, the miniaturization of band-pass filters used in this type ofcommunication equipment has been in high demand.

As compact band-pass filters, in the related art, surface acoustic wavefilters that utilize surface acoustic waves have been commonly used. InJapanese Unexamined Patent Application Publication No. 7-115343, asurface acoustic wave filter device capable of being miniaturized isdisclosed.

As shown in FIG. 7, in a method for manufacturing a surface acousticwave device 101 that is disclosed in Japanese Unexamined PatentApplication Publication No. 7-115343, first, a piezoelectric substrate102 is prepared. On the top surface of the piezoelectric substrate 102,electrodes, such as IDT electrodes 103, are arranged to define a surfaceacoustic wave filter. The IDT electrodes 103 are provided in a portionof the piezoelectric substrate. The portion of the piezoelectricsubstrate is excited when the surface acoustic wave filter is used, anda surface acoustic wave is propagated. Accordingly, it is necessary toform an air gap A that the IDT electrodes 103 face. For this reason, asshown in FIG. 7, a cover member 104 is bonded to the piezoelectricsubstrate 102 so that the cover member 104 covers the top surface of thepiezoelectric substrate 102 and so that the air gap A is maintained.

In the manufacturing method disclosed in Japanese Unexamined PatentApplication Publication No. 7-115343, the bonding of the cover member104 is performed by direct bonding without using an adhesive. Directbonding is a method in which heat is applied to the piezoelectricsubstrate 102 and the cover member 104 with the piezoelectric substrate102 being in contact with the cover member 104 to make hydrogen bondstherebetween, thereby directly bonding the piezoelectric substrate 102and the cover member 104 together. Accordingly, the surfaces of portionsfacing the surfaces of the piezoelectric substrate 102 and the covermember 104 which are to be bonded must include hydrophilic groups. Thehydrophilic groups are to be bonded by hydrogen bonds of water moleculesprovided from moisture.

Because the bonding is performed by direct bonding, no adhesive isrequired. A sealing property of portions of the cover member 104 and thepiezoelectric substrate 102 to be bonded can be increased.

Because, in the surface acoustic wave filter device 101 disclosed inJapanese Unexamined Patent Application Publication No. 7-115343, it isnecessary to form the air gap A, the size of the entire package isincreased.

Additionally, a recessed portion used to form the air gap A must beprovided in the cover member 104, or the cover member 104 having arecessed portion must be provided. Accordingly, there is also a problemthat the cost is high.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a surface acoustic wave device which does notrequire a complicated processing step, which does not requirepreparation of a component, such as a cover member having a complicatedshape, which is inexpensive, and which is capable of having a reducedsize, and a method for manufacturing the surface acoustic wave device.

A method for manufacturing a surface acoustic wave device according to apreferred embodiment of the present invention includes the steps ofpreparing a piezoelectric substrate and a cover member, forming, in oneprincipal surface of the piezoelectric substrate, grooves in portions inwhich an IDT electrode and a wiring electrode are to be provided,forming, on the one principal surface of the piezoelectric substrate, ametallic film at least in the grooves, forming a recessed portion byremoving one portion of the piezoelectric substrate from the principalsurface side of the piezoelectric substrate to form, using the metallicfilm located in the grooves, the IDT electrode and the wiring electrode,and bonding the cover member to the one principal surface of thepiezoelectric substrate.

The step of forming the recessed portion is preferably performing usinga chemical mechanical polishing (CMP) process from the one principalsurface side of the piezoelectric substrate.

The step of forming the recessed portion is preferably performed by dryetching while a mask is applied to the one principal surface of thepiezoelectric substrate.

The step of bonding the cover member to the piezoelectric substrate ispreferably performed by direct bonding.

When the piezoelectric substrate and the cover member are to be directlybonded together, a hydrophilic insulating thin film is preferably formedon the one principal surface of the piezoelectric substrate or on asurface of the cover member that is to be bonded to the piezoelectricsubstrate.

The method described above preferably further includes a step offorming, in the piezoelectric substrate, a through-hole electrode thatis to be connected to the wiring electrode.

The method described above preferably further includes a step offorming, in the cover member, a through-hole electrode that is to beelectrically connected to the wiring electrode.

In another preferred embodiment of the method for manufacturing asurface acoustic wave device according to the present invention, apiezoelectric substrate is provided as the cover member and includes anIDT electrode and a wiring electrode formed in one principal surfacethereof.

A surface acoustic wave device according to a preferred embodiment ofthe present invention includes a piezoelectric substrate having arecessed portion provided in one principal surface thereof, a groovehaving a planar shape corresponding to a shape of an IDT electrode isprovided in the recessed portion, an IDT electrode embedded in thegroove, a cover member bonded to the one principal surface of thepiezoelectric substrate, a through-hole electrode that is provided inthe piezoelectric substrate and/or the cover member, and that iselectrically connected to the IDT electrode, and a terminal electrodethat is provided in the piezoelectric substrate and/or the cover member,that is electrically connected to the through-hole electrode, and thatis disposed so as to be electrically connected to an external device.

A groove having a planar shape corresponding to a shape of a wiringelectrode is preferably provided in the one principal surface of thepiezoelectric substrate at a location that is different from that of therecessed portion, and the wiring electrode is embedded in the groove.

In the method for manufacturing a surface acoustic wave device accordingto a preferred embodiment of the present invention, the grooves areformed in the one principal surface of the piezoelectric substrate.Thereafter, the metallic film is formed at least in the grooves, and oneportion of the piezoelectric substrate is removed from the principalsurface side of the piezoelectric substrate to form the recessedportion. By forming the recessed portion, the IDT electrode and thewiring electrode are formed of the metallic film located in the grooves.Even when the metallic film extends to the periphery of the grooves,when the recessed portion is formed, the recessed portion is formed soas to remove a portion of the metallic film that is located at theperiphery of the grooves. Accordingly, in the one principal surface ofthe piezoelectric substrate, the IDT electrode is formed at a locationlower than that of the one principal surface. Thus, for example, evenwhen a cover member having a planar shape is used, by bonding the covermember to the one principal surface, an air gap that the IDT electrodefaces can be formed using the recessed portion. Therefore, no covermember having a complicated shape is required. Additionally, nocomplicated step of forming an air gap in the cover member is required.

When the recessed portion is formed, the size of the air gap can beprecisely and easily adjusted, and the air gap can be formed by formingthe recessed portion. Accordingly, the size of the surface acoustic wavedevice can be reduced. Thus, the cost and the size of the surfaceacoustic wave device can be reduced.

When the recessed portion is formed by performing a CMP process from theone principal surface side of the piezoelectric substrate, the recessedportion can be precisely and easily formed without degradation of thepiezoelectric substrate.

When the recessed portion is formed by dry etching while a mask isapplied, the recessed portion can be easily formed only by preparing amask having a shape corresponding to a planar shape of the recessedportion and by performing the dry etching.

When the step of bonding the cover member is performed by directbonding, no adhesive is required. Accordingly, the cost can be reduced.Additionally, when the cover member is bonded to the piezoelectricsubstrate by the direct bonding, a low profile is further enhanced.Furthermore, the surface acoustic wave device can be provided, in whichvariations in the bonded portion is reduced, in which variations incharacteristics are reduced, and which has an outstanding reliability.

When the piezoelectric substrate and the cover member are directlybonded together, a hydrophilic insulating thin film may be formed on theone principal surface of the piezoelectric substrate and/or the surfaceof the cover member that is to be bonded to the piezoelectric substrate.With this configuration, even when the piezoelectric substrate or thecover member does not include hydrophilic groups on the surface thereof,by forming the hydrophilic insulating thin film, the piezoelectricsubstrate and the cover member can securely bonded together by thedirect bonding.

When the method further includes the step of forming, in thepiezoelectric substrate, a through-hole electrode that is to beconnected to the wiring electrode, the wiring electrode can be, forexample, connected to an external device via the through-hole electrode.Similarly, when the method further includes the step of forming, in thecover member, a through-hole electrode that is to be electricallyconnected to the wiring electrode, the wiring electrode can be easilyelectrically connected to, for example, an external device via thethrough-hole electrode.

When the cover member is defined by a piezoelectric substrate includingan IDT electrode and a wiring electrode, a surface acoustic wave devicein which two surface acoustic wave elements are stacked can be providedin accordance with preferred embodiments of the present invention.Accordingly, electronic equipment or a circuit in which the surfaceacoustic wave device is mounted can be reduced in size and designed witha high density.

In the surface acoustic wave device according to preferred embodimentsof the present invention, the recessed portion is formed in the oneprincipal surface of the piezoelectric substrate. The groove having aplaner shape corresponding to a shape of a portion of an IDT electrodeis formed in the recessed portion, and the IDT electrode is formed so asto be embedded in the groove. Accordingly, the cover member is bonded tothe one principal surface of the piezoelectric substrate, and an air gapcan be formed which does not prevent the IDT electrode from vibrating inthe recessed portion. Thus, the size of the surface acoustic wave devicecan be reduced. Furthermore, because the air gap is formed using therecessed portion, a cover member having a planar shape can be used,thereby reducing the cost of the cover member.

The through-hole electrode may be provided in the piezoelectricsubstrate and/or the cover member, and the terminal electrode that iselectrically connected to the through-hole electrode and that isdisposed so as to be electrically connected to an external device may beprovided. In such a configuration, the IDT electrode can be easilyelectrically connected to the external device via the through-holeelectrode and the terminal electrode.

Accordingly, the surface acoustic wave device can be obtained, which isminiaturized, inexpensive, and arranged to be easily mounted on aprinted circuit board.

The groove having a planar shape corresponding to a shape of a wiringelectrode may be formed in the one principal surface of thepiezoelectric substrate at a location different from that of therecessed portion, and the wiring electrode may be embedded in thegroove. In such a configuration, the wiring electrode can be formed bythe same step as that of forming the IDT electrode. As a result, thesize and cost of the surface acoustic wave device can be reduced.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a front sectional view of a surface acoustic wavefilter device according to a preferred embodiment of the presentinvention and a partial enlarged front sectional view of the surfaceacoustic wave filter device, illustrating the main portion of thesurface acoustic wave filter device.

FIGS. 2A to 2E are schematic front sectional views of the surfaceacoustic wave filter device according to a preferred embodiment of thepresent invention, illustrating a method for manufacturing the surfaceacoustic wave filter device.

FIG. 3A to 3D are schematic front sectional views of the surfaceacoustic wave filter device according to a preferred embodiment of thepresent invention, illustrating the method for manufacturing the surfaceacoustic wave filter device.

FIGS. 4A to 4C are schematic front sectional views of a recessedportion, describing a modification example in which the recessed portionis formed by dry etching.

FIGS. 5A to 5C are schematic front sectional views of the recessedportion, describing the modification example in which the recessedportion is formed by dry etching.

FIG. 6 is a front sectional view of a surface acoustic wave filterdevice according to another preferred embodiment of the presentinvention.

FIG. 7 is a front sectional view of a surface acoustic wave filterdevice in the related art, describing an example of the surface acousticwave filter device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the drawings, whereby the present invention willbecome apparent.

FIGS. 1A and 1B are a front sectional view of a surface acoustic wavefilter device according to a preferred embodiment of the presentinvention and a partial enlarged front sectional view of the surfaceacoustic wave filter device, showing the enlarged main portion of thesurface acoustic wave filter device. A method for manufacturing asurface acoustic wave filter device 1 according to the preferredembodiment will be described with reference to FIG. 2A to 2E and FIGS.3A to 3D.

First, FIG. 2A, a piezoelectric substrate 2 is prepared. Thepiezoelectric substrate 2 may be made of a piezoelectric single crystalor a piezoelectric ceramic. In this preferred embodiment, thepiezoelectric substrate 2 is made of crystal. On a top surface 2 adefining one principal plane of the piezoelectric substrate 2, a mask 3is formed using a photolithography method. The resist mask 3 includesopenings defining portions in which IDT electrodes and wiring electrodesare to be formed. Instead of using the resist mask 3, a metallic maskmade of Ni, Cr, or other suitable materials may be used.

Next, as shown in FIG. 2B, portions of the piezoelectric substratefacing the openings of the resist mask 3 are removed using an etchingmethod to form grooves 2 b, 2 c, and 2 c. The grooves 2 b have a planarshape corresponding to a planar shape in which the IDT electrodes are tobe formed. The grooves 2 c and 2 c are provided in portions in which thewiring electrodes are to be formed.

When the etching is performed, an appropriate etchant that does notdissolve the resist mask 3 may be used. When the piezoelectric substrate2 is formed of crystal, a fluoride gas, such as CF₄ or SF₆, or achloride gas, such as BCl₃, may be used as the etchant. When thepiezoelectric substrate 2 is formed of LiTaO₃ or LiNbO₃, a fluoride gas,Ar, or other suitable etchant may be used.

Next, the resist mask 3 is dissolved and removed using a resist removalliquid. In this manner, as shown in FIG. 2C, the piezoelectric substrate2 in the top surface 2 a of which the grooves 2 b, 2 c, and 2 c areformed is prepared.

Then, as shown in FIG. 2D, a metallic film M is formed on the entire topsurface 2 a of the piezoelectric substrate 2 using an appropriatethin-film forming method, such as vapor deposition, sputtering, orplating, for example. Because the metallic film M functions as anelectrode constituent material, the metallic film M can be formed of anappropriate metallic material. For example, the metallic film M may beformed of Al, Cu, Au, Ta, or W, an alloy of Al, Cu, Au, Ta, or W, orstacked metallic films formed of two or more types of metals chosen fromamong Al, Cu, Au, Ta, and W, for example.

After that, as shown in FIG. 2E, using a CMP process, a portion of thetop surface 2 a of the piezoelectric substrate is removed from the topsurface 2 a side of the piezoelectric substrate 2 to form a recessedportion 2 d. The recessed portion 2 d is formed so that theabove-described grooves 2 b are located in the recessed portion 2 d,i.e., so that a region in which the IDT electrodes are to be formed islocated in the recessed portion 2 d. When the CMP process is performed,the recessed portion 2 d is formed, and a portion of the metallic filmthat is located at the periphery of a portion in which the IDTelectrodes 6 are formed is removed. Additionally, wiring electrodes 5and 7 are made of the metallic film that remains in the recessedportions 2 c and 2 c. A process of removing the portion of the topsurface 2 a of the piezoelectric substrate 2 by the CMP process isperformed so that the metallic film excluding the portions in which theIDT electrodes 6 and the wiring electrodes 5 and 7 are formed can beremoved. Accordingly, the IDT electrodes 6 are embedded in the grooves 2b. Similarly, the wiring electrodes 5 and 7 are embedded in the grooves2 c and 2 c, respectively.

The CMP process is commonly used as a polishing method used in, forexample, a method in which an interlayer-insulating film of asemiconductor device is formed. In the CMP process, when polishing isperformed by a polishing head, a polishing liquid is supplied. The CMPprocess is a process in which mechanical polishing performed by thepolishing head and chemical polishing using the polishing liquid areperformed together.

The parameters of the CMP process can be appropriately set in accordancewith the piezoelectric substrate 2 to be used, the depth and size of therecessed portion 2 d to be formed are not particularly limited. However,in this preferred embodiment, for example, the CMP process may beperformed by applying a polishing pressure of approximately 100 g/cm²using a polishing pad, which is configured using IC1000 manufactured byNitta Haas, rotating at several tens rpm to several hundred rpm, andsimultaneously by supplying a polishing liquid, for example. Thepolishing liquid, for example, is obtained by diluting one part ofPlanerlite 7102, which is manufactured by Fujimi Incorporated, with fiveparts of pure water.

When the above-described CMP process is performed, the recessed portion2 d is formed by erosion. The internal surface of the recessed portion 2d is formed as a curved surface around the edges of the recessed portion2 d. Additionally, the surface of the metallic film filling the grooves2 c is curved due to an effect of dishing as shown in FIG. 2E. In otherwords, since the central portions are polished more than the edgeportions, the top surfaces of the wiring electrodes 5 and 7 are curved.

Next, as shown in FIG. 3A, through holes 2 f and 2 g are formed, whichare used to form through-hole electrodes so that the through-holeelectrodes can extend to the wiring electrodes 5 and 7 formed in the topsurface 2 a from a bottom surface 2 e of the piezoelectric substrate 2.The process of forming the through holes 2 f and 2 g is not particularlylimited. However, the through holes 2 f and 2 g can be formed using anappropriate processing method in which a laser or sandblasting is used,for example. For example, a mask is formed on the bottom surface 2 e ofthe piezoelectric substrate 2, and the mask has openings definingportions in which the through holes 2 f and 2 g are to be formed. Byperforming a sandblasting process, the through holes 2 f and 2 g can beformed.

Then, as shown in FIG. 3B, the through holes 2 f and 2 g are filled witha metallic material, for example, by plating, thereby formingthrough-hole electrodes 9 and 10.

After that, as shown in FIG. 3C, terminal electrodes 11 and 12 areformed on the bottom surface 2 e of the piezoelectric substrate 2 sothat the terminal electrodes 11 and 12 can be electrically connected tothe through-hole electrodes 9 and 10, respectively. The terminalelectrodes 11 and 12 can be formed using an appropriate method such asplating or printing of metal bumps, for example.

Then, as shown in FIG. 3D, a cover member 13 having a planar shape isbonded to the top surface of the piezoelectric substrate 2. In thiscase, for the cover member 13, an appropriate insulating material suchas crystal, glass, or an Si-based material, for example, may be used.Preferably, the cover member 13 includes hydrophilic groups on thesurface thereof.

After that, by direct bonding, the cover member 13 is bonded to the topsurface 2 a of the piezoelectric substrate 2. The direct bonding can beperformed by, for example, application of heat at about 200° C. to about400° C. for about one hour. As disclosed in Japanese Unexamined PatentApplication Publication No. 7-115343, which is described above, thedirect bonding is a method in which the cover member 13 and thepiezoelectric substrate 2 are bonded together by hydrogen bonds withoutusing an adhesive.

In other words, where hydrophilic groups exist on surfaces of thepiezoelectric substrate 2 and the cover member 13 that are to be bonded,when heat is applied, water molecules provided from moisture in theatmosphere are bonded to the hydrophilic groups. Accordingly, thepiezoelectric substrate 2 and the cover member 13 are bonded together byhydrogen bonds.

Before the direct bonding is performed, the surface of the piezoelectricsubstrate 2 may be immersed in a liquid mixture of ammonia and hydrogenperoxide in advance, thereby performing a process of making the surfacehydrophilic.

When the surfaces of the cover member 13 and the piezoelectric substrate2 do not include hydrophilic groups, preferably, a hydrophilicinsulating thin film may be formed on the surface of the piezoelectricsubstrate 2 and/or the surface of the cover member 13 to be bonded.

By using the direct bonding method, in substantially the entire regionof portions at which the cover member and the piezoelectric substrate 2are in contact with each other and bonded together, the cover member 13and the piezoelectric substrate 2 are strongly bonded together.Accordingly, the recessed portion 2 d can be securely sealed in anairtight manner. The reliability of the surface acoustic wave device canalso be increased, accordingly.

In the present preferred embodiment, when, to adjust the temperaturecharacteristics or the frequency characteristics, a dielectric film ofSiO₂, SiN, or other suitable material is provided on the IDT electrodes,the dielectric film may be arranged over substantially the entiresurface of the piezoelectric substrate. After a process of making thedielectric film hydrophilic is appropriately performed, the directbonding is performed.

In this manner, the surface acoustic wave filter device 1 shown in FIGS.1A and 1B can be obtained.

As clearly shown in FIGS. 1A and 1B, in this present preferredembodiment, the recessed portion 2 d is formed by the CMP process,thereby providing an air gap that the IDT electrodes 6 face.Accordingly, the cover member 13 having a planar shape, i.e., a covermember at least the bottom surface of which has a planar shape, can beused. A cover member having a complicated shape is not required. Inaddition, a complicated process is not required when forming the covermember. Thus, the cost of the surface acoustic wave filter device 1 canbe reduced.

Additionally, by adjusting process conditions in which the CMP processis performed, the recessed portion 2 d can be precisely and easilyformed. Accordingly, the depth of the recessed portion 2 d can also bereduced. Thus, the size of the surface acoustic wave filter device 1 canbe minimized. More specifically, a low profile can be achieved.

Furthermore, the bonding is performed by the direct bonding, and noadhesive is used. Accordingly, characteristic deterioration caused by avariation in the bonded portion is not likely to occur. Accordingly, thelow profile can be further improved.

In this preferred embodiment, the wiring electrodes are connected to theterminal electrodes via the through-hole electrodes. However, withoutproviding the through-hole electrodes, the wiring electrodes may beelectrically connected to the terminal electrodes via wiring patternsthat are provided on the external surface of the piezoelectricsubstrate.

Furthermore, the through-hole electrodes may be provided on the covermember 13 side, instead of on the piezoelectric substrate side.

In this preferred embodiment, the recessed portion 2 d is formed by theCMP process, whereby the air gap that the IDT electrodes face is formed.However, a dry etching method may be used when a recessed portion thatfaces IDT is formed. The IDT electrodes face the recessed portion. As amodified example, a method for forming the recessed portion using thedry etching method will be described with reference to FIG. 4A to 4C andFIGS. 5A to 5C.

In the modified example, as schematically shown in FIG. 4A, a mask 22 isformed on a top surface 21 a of a piezoelectric substrate 21. The mask22 includes openings 22 a in which IDT electrodes and wiring electrodes(not shown) are to be formed.

In other words, as shown in FIG. 4B, grooves 21 b are formed in the topsurface 21 a defining one principal plane of the piezoelectric substrate21 using the dry etching method. The grooves 21 b have a planar shapecorresponding to a planar shape of the IDT electrodes.

After that, as shown in FIG. 4C, a metallic film 23 forming the IDTelectrodes is provided over the entire surface, thereby filling thegrooves 21 b with the metallic film to form the metallic film 23defining the IDT electrodes.

Then, the mask 22 and the metallic film 23, which is stacked on the mask22, are removed using a lift-off method using a solvent that removes themask 22. In this manner, as shown in FIG. 5A, the piezoelectricsubstrate 21 is obtained, which includes the metallic film 23 definingthe IDT electrodes that are embedded in the grooves 21 b.

The mask 22 may be formed of a material similar to that of the resistmask 3 or the metallic film M in the forgoing preferred embodiment.Additionally, regarding the method for forming the grooves 21 b by dryetching, the method can be performed by dry etching using an appropriateetchant in accordance with the material of the piezoelectric substrate21.

Next, as shown in FIG. 5B, another mask 25 is disposed on the topsurface 21 a of the piezoelectric substrate 21. The mask 25 includes anopening in which the metallic film 23 defining the IDT electrodes isformed. When the mask 25 is disposed, one portion of the top surface 21a of the piezoelectric substrate 21 and the upper portion of themetallic film 23 defining the IDT electrodes are removed using the dryetching method to form a recessed portion 21 c. Thereafter, the mask 25is removed by a solvent. In this manner, as shown in FIG. 5C, thepiezoelectric substrate 21 that includes the recessed portion 21 c inthe top surface 21 a can be obtained. In the recessed portion 21 c, IDTelectrodes 24 are disposed.

FIGS. 4A to 5C schematically show the method for manufacturing a portionin which the IDT electrodes 24 are to be formed. However, a portion inwhich the wiring electrodes are to be formed is also simultaneouslyformed in the same manner. After the recessed portion 21 c is formed asshown in FIG. 5C, through-hole electrodes and terminal electrodes areformed as in forgoing preferred embodiment, and then a cover member isbonded. In this manner, a surface acoustic wave filter device isproduced.

FIG. 6 is a front sectional view of a surface acoustic wave filterdevice according to another preferred embodiment of the presentinvention. In the surface acoustic wave filter device 1 shown in FIG. 1,the cover member 13 is bonded to the top surface 2 a of thepiezoelectric substrate 2. However, in a surface acoustic wave filterdevice 31 shown in FIG. 6, another upside-down piezoelectric substrate 2is stacked on the top surface of the piezoelectric substrate 2, andbonded by direct bonding. In other words, instead of using the covermember 13 according to the first preferred embodiment, anotherpiezoelectric substrate 2 having a configuration similar to that of thepiezoelectric substrate 2 is used. The principal planes of the pair ofthe piezoelectric substrates 2 and 2 in which the IDT electrodes 6 areformed are stacked together, and the pair of the piezoelectricsubstrates 2 and 2 are bonded together by the direct bonding.

Accordingly, in this preferred embodiment, a stacked-type surfaceacoustic wave filter device 31 is formed, in which two surface acousticwave filter elements are stacked.

On the top surface of the upper piezoelectric substrate 2, terminalelectrodes 32 and 33 that are connected to the through-hole electrodes 9and 10, respectively, are formed. The terminal electrodes 32 and 33extend to the bottom surface of the lower piezoelectric substrate viathe side surfaces of the stacked-type device in which the piezoelectricsubstrates 2 and 2 are stacked. On the bottom surface of the lowerpiezoelectric substrate 2, terminal electrodes 34 and 35 that areconnected to terminal electrodes 32 and 33, respectively, are formed.Accordingly, the surface acoustic wave filter device 31 can besurface-mounted on a circuit board using the terminal electrodes 34 and35 and the terminal electrodes 11 and 12.

In the present invention, the cover member does not necessarily functionas a casing material. The cover member may function as an electroniccomponent element.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A method for manufacturing a surface acoustic wave device comprisingthe steps of: preparing a piezoelectric substrate and a cover member;forming, in one principal surface of the piezoelectric substrate,grooves in portions in which an IDT electrode and a wiring electrode areto be provided; forming, on the one principal surface of thepiezoelectric substrate, a metallic film at least in the grooves;forming a recessed portion by removing one portion of the piezoelectricsubstrate from the principal surface side of the piezoelectric substrateto form, using the metallic film positioned in the grooves, the IDTelectrode and the wiring electrode; and bonding the cover member to theone principal surface of the piezoelectric substrate.
 2. The method formanufacturing a surface acoustic wave device according to claim 1,wherein the step of forming the recessed portion is performed by a CMPprocess from the one principal surface side of the piezoelectricsubstrate.
 3. The method for manufacturing a surface acoustic wavedevice according to claim 1, wherein the step of forming the recessedportion is performed by dry etching while a mask is applied to the oneprincipal surface of the piezoelectric substrate.
 4. The method formanufacturing a surface acoustic wave device according to claim 1,wherein the step of bonding the cover member to the piezoelectricsubstrate is performed by direct bonding.
 5. The method formanufacturing a surface acoustic wave device according to claim 4,wherein, when the piezoelectric substrate and the cover member are to bedirectly bonded together, a hydrophilic insulating thin film is formedon the one principal surface of the piezoelectric substrate or on asurface of the cover member that is to be bonded to the piezoelectricsubstrate.
 6. The method for manufacturing a surface acoustic wavedevice according to claim 1, further comprising a step of forming, inthe piezoelectric substrate, a through-hole electrode that is to beconnected to the wiring electrode.
 7. The method for manufacturing asurface acoustic wave device according to claim 1, further comprising astep of forming, in the cover member, a through-hole electrode that isto be electrically connected to the wiring electrode.
 8. The method formanufacturing a surface acoustic wave device according to claim 1,wherein the cover member includes a piezoelectric substrate in oneprincipal surface of which an IDT electrode and a wiring electrode areformed.
 9. A surface acoustic wave device comprising: a piezoelectricsubstrate including a recessed portion in one principal surface thereofand a groove having a planar shape corresponding to a shape of an IDTelectrode provided in the recessed portion; an IDT electrode embedded inthe groove; a cover member bonded to the one principal surface of thepiezoelectric substrate; a through-hole electrode provided in one of thepiezoelectric substrate and the cover member, and being electricallyconnected to the IDT electrode; and a terminal electrode provided in oneof the piezoelectric substrate and the cover member, that iselectrically connected to the through-hole electrode, and that isarranged so as to be electrically connected to an external device. 10.The surface acoustic wave device according to claim 9, wherein a groovehaving a planar shape corresponding to a shape of a wiring electrode isprovided in the one principal surface of the piezoelectric substrate ata location different from that of the recessed portion, and the wiringelectrode is embedded in the groove.